1. Technical Field
The invention relates to a technology for reading out the output of a light-receiving element, such as a photodiode.
2. Related Art
In recent years, display panels in which liquid crystal elements or organic EL (electroluminescent) elements are arranged in a matrix in electronic apparatuses, such as cellular phones and PDAs (personal digital assistants) have been widely spread. Such a display panel is used in various environments from a bright state to a dark state. For this reason, when the brightness or image quality of a display element is uniform irrespective of external light, it is easy for a user to see a display screen in any environment. On the other hand, it is very difficult for the user to see the display screen in other environments. Therefore, in the display panel, it is required that the amount of external light be detected and that the brightness or image quality of a display image be controlled according to the detected result.
In the control of the display image, the light receiving element, such as a photodiode, is used for detecting the amount of light. However, when the light receiving element is separately provided from the display panel, problems arise in that a space for mounting the light receiving element in an electronic apparatus is needed, or it is required to form an opening for detecting external light outside of the electronic apparatus.
In order to solve the above-mentioned problems, a technology in which the light receiving element is formed by the same process as a thin film transistor (hereinafter, referred to as a ‘TFT’) for switching a pixel on the display panel and that the external light is detected from the display panel has been considered.
According to this technology, noise becomes mixed into an output line to thus deteriorate the detection precision of the amount of light. Therefore, there has been suggested a technology in which the noise generated in the output line is detected, the detected noise is inverted to be supplied to the output line, and the noise generated in the output line is corrected (see Japanese Unexamined Patent Application Publication No. 9-82931 (FIG. 1) which is an example of the related art).
However, according to the above-mentioned technologies, since the output signal is an analog signal, it is difficult to perform the subsequent processes. In order to solve this problem, the output of the light receiving element may be A/D converted, but it is not easy to constitute an A/D converter with TFTs. As a result, the A/D converter must be separately provided from the display panel, and thus, the external light cannot be detected by the display panel as is.
In addition, the display panel is used under various environments from a very bright state, as in sunlight (about, 100,000 lux), to a very dark state in which a color is seen (about, 10 lux). As a result, it is required that the detection range of the amount of received light in the optical sensor provided on the display panel be very wide.